1. Field of the Invention
The present invention relates to a process for manufacturing chopped thermoplastic fibers, especially chopped glass fibers, in which the thermoplastic fibers are chopped in a region where at least an anvil wheel and a blade-holder wheel, rotating simultaneously, are in contact with each other. Although not limited to such an application, the invention will more particularly be described with reference to the manufacture of chopped glass fibers extruded directly beneath bushings.
2. Description of the Related Art
For this type of application, it is well known to cut the glass fibers continuously by using a chopping assembly consisting of two contacting wheels, one called the anvil wheel and the other the blade-holder wheel. The chopping assembly must both draw several continuous glass fibers coming from bushings at a high rate of about one or more tens of meters per second and chop each glass fiber into fragments of predetermined length.
A major problem encountered during such manufacture is associated with the phenomenon of relative wear of the contacting wheels, this wear being greater the higher their speed of rotation and the higher the contact pressure between them. In particular, wear occurs rapidly and progressively in both the cutting edge of the blades and on the external peripheral surface of the anvil wheel made of an elastomer of the polyurethane type.
Such wear means that the contact between the two wheels becomes less and less suited to drawing the glass fibers correctly and that the deformation applied by each of the blades to the external peripheral surface of the anvil wheel no longer allows the fibers to be broken cleanly. Consequently, the quality of the produced chopped fibers deteriorates progressively.
Hitherto, this difficulty has been overcome by accepting a somewhat degraded quality of chopped fiber, and when this degradation is excessive, by frequently stopping the production so as to carry out a wheel change. Quite obviously, this change is detrimental to the production yield.
It is an object of the invention to improve the production yield of chopped fibers of the type mentioned above without thereby degrading their quality.
To achieve this and other objects, the invention comprises a is process for manufacturing chopped thermoplastic fibers, especially chopped glass fibers, in which the thermoplastic fibers are chopped in a region where at least an anvil wheel and a blade-holder wheel, rotating simultaneously, are in contact with each other. At least part of the peripheral surface of the rotating anvil wheel in contact with the blade-holder wheel is machined so as to compensate for its wear, while maintaining the anvil wheel in contact with the blade-holder wheel.
The machining reduces the thickness of the anvil wheel, preferably by grinding the peripheral surface of the wheel.
The machining of the invention is a simple and effective solution to the conventional problems. To achieve this solution, the inventors have determined that the wheel thickness change was necessitated only for the anvil wheel, the wear of the material (polyurethane) of whose external peripheral surface is high, while the wear of the cutting edge of the blades was lower. They have discarded the solution of using a more wear-resistant material for the anvil wheel, this solution being tricky to develop and difficult to apply under industrial conditions.
There are many advantages provided by the invention. First, because of the direct machining, the frequency of wheel changes is considerably reduced, hence a significant increase in production time. Moreover, this increase is all the greater since the production is not interrupted during direct machining.
Next, the costs associated with equipment consumption are also considerably reduced. In fact, according to the prior art, the cost of consumables associated with frequently reconditioning the wheels was far from negligible, especially by frequently putting the anvil wheel on a conventional machining device, such as a lathe, etc.
Furthermore, the quality of the chopped fiber obtained is much more constant.
In addition, direct in-situ machining makes it possible to ensure that there is always perfect coaxiality between the external peripheral surface of the anvil wheel and the rotation spindle which supports it, something which might not conventionally be the case insofar as, especially when refitting the anvil wheel whose wear had been compensated for on a conventional machining device, there was always a risk of the axis of symmetry of this wheel not being coincident with the rotation spindle.
Finally, regenerating the external peripheral surface of the anvil wheel results in a reduction in the wear of the blades, this wear being much less since they no longer suffer abrasion due to the thermoplastic particles resulting from the chopping, which particles are no longer embedded in the machined surface.
According to an advantageous characteristic of the invention, the fibers are drawn using the assembly formed by the anvil wheel and the blade-holder wheel.
Preferably, prior to chopping, the thermoplastic fibers are arranged in such a way that they bear on part of the peripheral surface of one of the wheels, preferably the anvil wheel. Such an arrangement helps in the frictional drawing and entrainment of the fibers on the anvil wheel.
Preferably still, the thermoplastic fibers are obtained by a direct fiberizing process.
The invention also relates to an apparatus for implementing the process that has just been described. This apparatus is noteworthy in that it comprises at least an anvil wheel and a blade-holder wheel in contact with each other; and a device for machining at least part of the external peripheral surface of the anvil wheel.
Advantageously, the apparatus also comprises a device for arranging the fibers such that they bear on part of the peripheral surface of one of the wheels, preferably the anvil wheel.
According to another characteristic of the invention, the anvil wheel consists of a roll covered over at least part of its circumference with a covering made of a polymer material, especially an elastomer of the polyurethane type.
Advantageously, the device for carrying out the machining of the aforementioned apparatus comprise at least one abrasive wheel. Of course, the device for carrying out the machining may comprise a cutting tool such as a blade.
However, the abrasive wheel is perfectly suited to the machining according to the invention insofar as it constitutes a simple and compact tool capable of xe2x80x9cregenerating,xe2x80x9d that is making uniform again, the external peripheral surface of the anvil wheel running with a high peripheral velocity.
According to an additional characteristic, the abrasive wheel consists of a cylinder, preferably made of metal, the peripheral surface of which is covered with a multitude of grit particles of the diamond type. This particular grinding-wheel structure makes it possible to abrade without the risk of slip between the surfaces in question.
According to another characteristic, the movement of the abrasive wheel must allow the grinding wheel to machine it over the entire peripheral surface simultaneously, working in a so-called plunge-grinding mode.
In order to further optimize the work of the machining device, the operation and, where necessary, the movement of the device for carrying out the machining is responsive to a controller receiving inputs from, e.g., a device for checking the external peripheral surface finish of the anvil wheel, such as an optical sensor or a roughness measurement sensor, or to a device for checking the quality of the chopped fibers.
In order to avoid any risk of the particles resulting from the machining fouling the various surrounding elements and, as the case may be, getting mixed up with the end-product, it is preferable for the apparatus to furthermore comprise a device for collecting them. These particles may either be xe2x80x9cchipsxe2x80x9d coming from particles of elastomer or may be abrasive particles liberated from the abrasive wheel.
The recovery device may preferably consist of at least one suction nozzle, preferably located immediately downstream of the device for carrying out the machining. The term xe2x80x9cdownstreamxe2x80x9d should be understood to mean here the relative position with respect to the direction of rotation of the anvil wheel. In fact, the position of the suction nozzle will be advantageously chosen depending on the main direction of ejection of the particles resulting from the machining. This direction will, of course, depend on the direction of relative rotation of the grinding wheel with respect to that of the anvil wheel, as well as on its relative velocity.
As mentioned previously, the invention is particularly applicable to the manufacture of chopped glass fibers extruded directly beneath bushings, especially those having a fiber diameter of between 5 and 24 xcexcm and/or those having a length of between 1.5 and 15 mm.